Combustion at high temperature leads to the formation of NOx, or oxides of nitrogen, because of the combination of oxygen with nitrogen at high temperature. This is a notorious pollutant and much effort is being put forth to reduce the formation of NOx.
One solution has been to premix the fuel with excess air whereby all of the combustion occurs with a local high excess air and therefore at a relatively low temperature. Such combustion, however, can lead to instability and incomplete combustion.
This problem is exacerbated in gas turbine engines. Once the proper lean mix is set for proper full load operation, low load operation must be considered. At decreasing loads the airflow decreases less than the fuel flow, leading to even leaner mixtures. The air temperature also decreases. Accordingly, flame stability and combustion efficiency (percentage of fuel burnt) becomes an increasing problem.